The present invention concerns a data storage apparatus with means for storing and retrieving data in respective write and read operations, wherein data are stored in a non-volatile manner as polarization states in ferroelectric or electret memory cells, wherein the memory cells are provided in a passive matrix-addressable array, wherein the memory cells can be individually addressed electrically for write and read operations, the addressing of the memory cell taking place via crossing electrodes of a respectively first and second electrode sets, wherein each electrode set comprises parallel stripe-like electrodes such that the electrodes of the first set substantially are oriented orthogonally to the electrodes of the second set; a method for performing a non-destructive readout in the apparatus according to claim 1; and a method for poling pairs of piezoelectric storage cells for write operations in the apparatus according to claim 1.
Memory devices based on ferroelectric or electret polymer material have many advantages, like small cell footprint, stacking of memory layers, adaptability for reel-to-reel production/inkjet based production, low temperature processing, low cost, etc. Typically, data are read from the memory by so-called destructive readout, where reversal of polarization during the read operation makes it necessary to write back the information for further storage in the memory. A destructive read mode of this kind is undesirable for many applications; for instance, the number of fatigue cycles related to read is doubled. The latter may present a problem, since ferroelectric or electret based memories only can withstand a finite number of write cycles. Furthermore, having to switch the polymer film twice means that the power consumption is increased, as is the complexity of the required circuitry enabling such write back.
The destructive read itself is a rather slow process (typically hundreds of nanoseconds to several microseconds, subject to applied field). This implies that with destructive read the polymer memory is not suitable for a number of major applications, such as replacing fast cache SRAM memory. Finally, when cells are reduced in size, the reduced signal strength may make it difficult to implement polymer memory in state of the art designs, or use at all with ultra-dense lithography ( less than 0.15 um).
It is well-known that many ferroelectric and electret materials, including both the well-known PVDF (polyvinylidene-difluoride) and the copolymer PVDF-TrFE (poly(vinylidene difluoride- trifluoroethylene)) have piezoelectric properties. In that connection there can be referred to Sessler and Gerhard-Multhaupt (editors) Electrets, Vol. 1 and Vol. 2, Chapt. 5, 8, 11 and 12 (3rd ed., Laplacian Press (1998)).
In European patent application EP-A2-0166 938 (Eiling and al.) which has a priority date of Jun. 1, 1984, there is disclosed a write and read method in combination with the data storage device based on a ferroelectric polymer memory layer, with the aim of increasing storage density and reducing access time. To this end data are stored in memory cells formed by a single domain in the polymer memory material. These domains are coded by being permanently polarized by an applied electric field and coded in accordance with the direction of the field, i.e. with a positive or negative polarization in the memory layer. For reading the data this application discloses the use of pyroelectric or piezoelectric activation of the polarized domains. This activation is envisaged to take place over the suitably arranged electrode means in a matrix-addressable memory cell array in a ferroelectric memory material, preferably e.g. poly(vinylidene difluoride-trifluoroethylene) (PVDF-TrFE), such that activation of a memory cell would take place either pyroelectrically or piezoelectrically, respectively by means of an applied heating current pulse or by applied pressure or tension.
Employing matrix-addressing, the proposed write and-read methods according to EP-A2-0168 938 moreover would allow a parallel write/read. An attainable storage density of 108 bit/cm2 is stated. It is indicated that the access time for a pyroelectric read would be of the order 10xe2x88x929 s. No indication is given for attainable access times for readout based on piezoelectric effects. Furthermore there is no indication in this application of the attainable performance in the small-signal range, neither with regard to access speed, applicable voltage levels, frequencies, detection reliability and noise immunity. However, since it is envisaged that non-destructive readout methods for memories based on ferroelectric polymers may operate in the small-signal range, particularly attainable signal/noise ratios and detection levels will be of major concern.
Hence it is a major object of the present invention to provide a data-storage apparatus allowing non-destructive read operations based on piezoelectric effects that can be obtained in a ferroelectric or electret memory material.
It is also a major object of the present invention to provide a very fast non-destructive readout method based on piezoelectric effects and with much improved signal/noise ratio as compared with prior art.
The above objects as well as further features and advantages are achieved with a data storage apparatus according to the present invention which is characterized in that the first set of electrodes comprises a plurality of parallel trenches extending horizontally with an orthogonal orientation to the electrodes and vertically downwards from a top surface thereof for a distance less than the thickness of the electrodes and having a substantially rectangular profile, that at least a ferroelectric or electret memory material is provided in the trenches and covering the sidewalls thereof, that the second set of electrodes is provided as a dual set of electrodes, each comprising parallel twin electrodes provided in said trenches and parallel therewith and without mutual contact with the electrodes of said first set, that the ferroelectric material is provided in at least a portion of the volume of the trenches not occupied by the electrodes, said ferroelectric or electret material being a soft or elastic ferroelectric or electret material with piezoelectric properties and in addition providing electrical insulation between the electrodes of the second set and the electrodes of the first set, whereby the twin electrodes of the second set under a suitably applied force can be displaced laterally in the soft ferroelectric or electret material with piezoelectric properties such that a piezoelectric effect is generated therein, and that said soft ferroelectric or electret material with piezoelectric properties in the volumes between the twin electrodes of the second set and the electrodes of the first set forms subcells of the memory cells in said trenches, said subcells capable of being poled for a write operation by applying an electric field between respective twin electrodes and crossing electrodes of the first set, such that the memory cells are polarized to store at least one of two or more logical values in the form of a set of polarization states in said subcells and can be read non-destructively by applying voltage potential between the twin electrodes, and/or between at least one of the latter and at least one of proximal electrodes of the first electrode set, and over the ferroelectric or electret material of the subcells which are subjected to tensile and/or compressive stresses enabling the detection of parameters deriving from the polarization states thereof by means of piezoelectric effects in response to strains thus induced in said subcells, whereby the logic state of the subcells of a memory cell can be found and the logical value or values stored in a memory cell containing the subcells can be determined.
The above objects as well as further features and advantages are also achieved with a method which is characterized by the readout of data being performed by subjecting the subcells of the memory cells selected for read operations to tensile and/or compressive force and concurrent detection of phase and/or polarity and/or magnitude of piezoelectrically generated voltage or current from said subcells in response to strains resulting from the application of said tensile and/or compressive force.
Finally, the above objects as well as further features and advantages are achieved with a method for poling pairs of piezoelectric memory cells for a write/read operation in the apparatus according to the invention and with the poling method characterized by poling each cell of the pair in one of two poling directions oriented orthogonally relative to the twin electrodes opposing each cell of the pair, whereby all four possible permutations of the poling directions can be realized in the two cells of the pair.
Further features and advantages of the present invention shall be apparent from the appended dependent claims.